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Living Colors And New Renewable Energy Sources: plant metabolites in solar cells

Many biological molecules give color to living objects by absorbing light of particular colors and
reflecting the remaining light. One of the best known examples is chlorophyll: an extremely important
biomolecule critical in photosynthesis that allows plants to absorb energy from light. Several types
of chlorophyll and closely related porphyrin-based structures are found in green pigments in
cyanobacteria and the chloroplasts of algae and plants. The function of the reaction center of
chlorophyll is to use the energy absorbed by and transferred to it from the other groups in the
photosystem to donate an electron into a series of molecular intermediates called an electron
transport chain.

The other significant group of plant dyes consists of mixes of anthocyanins belonging to a parent
class of flavonoids. In photosynthetic tissues (such as leaves and sometimes stems), anthocyanins
have been shown to act as a "sunscreen", protecting cells from high-light damage, by absorbing
blue-green and ultraviolet light and protecting the tissues from photoinhibition, or high-light
stress.

In biophotovoltaic dye-sensitized solar cells the dye-sensitizer mimics nature in absorbing
sunlight and transforming solar energy into electricity. A modern biomimetic solar cell usually
consists of arrays of titania nanocrystals coated with light-sensitive natural or synthetic pigments
playing the role equivalent to chlorophyll in nature. The many benefits of DSCs include lower purity
requirements and abundance of component materials, as well as the fact that they can be produced on
flexible substrates, making them amenable to roll-to-roll printing processes. Their advantages are
based on low cost of production, ease of fabrication and modifiable aesthetic features, such as
color and transparency [1]. DSCs are emerging as one of the most promising
low cost photovoltaic technologies, addressing secure, clean, and efficient solar energy conversion.

Chlorophyll's and anthocyanin's abitily to convert light energy into electrical energy makes them
prime candidates for use in DSC dye-sensitiers. In a natural solar cell the chlorophyll molecules
absorb light most strongly in the red and blue parts of the spectrum, leaving the green light to be
reflected. The absorbed energy is sufficient to free an electron from the excited chlorophyll.
Chlorophyll, cyanin, and carotene are several of the natural dyes that have been succesfully
utilized in DSCs. Others include phthalocyanine and porphyrin dyes developed based on natural prototypes.
Numerous metal complexes and organic dyes have been synthesized, many of them mimic chlorophyll-like
structures (for examples zinc-porphyrin-based light sensitizers).

Porphyrins are water-soluble biological pigments widely spread in animal and plant tissues. They
take part in important biological functions such as metal-binding cofactors in animal hemoglobins,
chlorophyll for photosynthesis, and certain enzymes for cell respiration. One of the best-known
porphyrins is heme, the pigment in red blood cells, a cofactor of the protein hemoglobin. The
specific porphyrin in heme B is called protoporphyrin IX and has 4 methyl, two vinyl, and two
propionic acid substituents.

Numerous types of porphyrins can be obtained through the modification of a natural porphyrins or
through chemical synthesis. Not only their chemical transformations through nucleophilic and
electrophilic substitution, substituent modification, reduction, oxidation but also their complex
formations with iron, zinc, copper, nickel and cobalt yield valuable biological applications in
molecular biology, fluoroimmunoassay and new materials science.

Compounds similar to porphin are the parent compounds for similar ring systems with other central
metal atoms in biochemistry. These include chlorin, which has a magnesium atom in several types of
chlorophyll; bacteriochlorin, found in bacteriochlorophyll; and corrin, which has a cobalt atom in
cobalamin or vitamin B12.

Another class of molecules with technological applications in biophotovoltaics is phenolic metabolites,
e.g. natural flavonoids and xanthonoids. Twenty natural dyes extracted from flowers, leaves, fruits, and
traditional Chinese medicinal herbs, have been successfully used as sensitizers in DSCs [2].
The list includes natural dyes extracted from begonia, tangerine peels, rhododendron, yellow rose,
petunia, mangosteen pericarp, coffee and other common plant materials. A dye mix of particular interest
obtained from mangosteen pericarp was found to contain xanthonoid mangostin and flavonoid-based rutin.

Rutin is a plant pigment that is found in several fruits and vegetables. Rutin is used to make
medicine. The major sources of rutin for medical use include buckwheat, Japanese pagoda tree, and
eucalyptus, lime tree flowers and elder flowers and hawthorn leaves. Anthocyanins and betalains from
many plant sources were also established as promising light-harvesting substances for dye-sensitized
solar cells [3].

Sources of natural dyes applied in solar cells development and production (from [3])

In various scientific studies related to the subject, NMR spectroscopy is often used as a powerful
tool in elucidating the structure of color-inducing metabolites, particularly to follow the
structural changes in the natural compound mixes. In modern structural studies of natural dyes, a
complete 1- and 2D 1H NMR and 13CNMR experiment set is a method of choice. The metabolomics
database at the BMRB contains 1H, 13C, 13CDEPT90, DEPT135, TOCSY, COSY45, HSQC, HMBC and
HSQC-TOCSY-ADIA NMR spectral data for various carotenoids, flavonoids and porphyrins, including such
derivatives as
zinc tetraphenylporphine,
protoporphyrin IX, and
rutin. They are presented as the time domain data, the spectra images, and the tables of assigned chemical shifts.